1. Field of the Invention
The present invention relates to a switching device in a digital unit of a multi-sector base station in a code division multiple access (CDMA) mobile communication system, and more particularly to a switching device in a digital unit of a multi-sector base station, which switching device uses switching elements incorporated in the digital unit and adapted to switch 3 sectors into multiple sectors, thereby being capable of implementing a multi-sector base station system allowing a softer handoff among all sectors.
2. Description of the Related Art
Recently, it has been generalized to use, rather than mono media consisting of single media, a plurality of mono media, by virtue of continued developments in performances of representation media, transfer media, and operating systems thereof, as well as continued developments in data compression techniques for storage media and transfer media. For this reason, future radio communication systems must support high-speed multimedia services associated with not only audio data, but also high-speed data and image data. In order to allow radio communication systems to accommodate high-speed multimedia services, it is, first of all, necessary to provide radio channels of a large capacity because those radio communication systems have limited frequency resources. For this reason, introduction of the concept of micro cells, which have a cell radius of several hundred meters, or pico cells, which have a cell radius of several ten meters, has been highlighted in order to efficiently reuse limited frequency resources.
In the case of micro cell systems, however, there may be a problem in that since those micro cell systems have a cell size considerably smaller than those of macro cell systems, handoff occurs frequently between adjacent base stations, thereby resulting in a degradation in speech quality. Handoff occurring between adjacent base stations in a cellular system is called xe2x80x9csoft handoffxe2x80x9d. This soft handoff is conducted using channel resources allocated to both base stations, thereby resulting in a waste of channel resources. Furthermore, a lengthened period of time is taken to process handoff because it is necessary to establish a channel for a new base station. On the other hand, xe2x80x9csofter handoffxe2x80x9d is a handoff occurring between adjacent sectors in a 3-sector base station. Accordingly, it is unnecessary to allocate additional channel elements (CEs). Also, a reduce handoff time is taken for the softer handoff, as compared to the soft handoff. In order to implement a micro cell system using a plurality of micro cells, therefore, it is necessary to design that system in such a fashion that softer handoff is made between adjacent cells. It is also necessary to develop a system capable of managing a plurality of cells in a centralized fashion.
A multi-sector base station system is a base station system capable of satisfying the above mentioned requirements.
Conventional base station systems are configured to have a maximum of three sectors. For this reason, a multi-sector base station system having a multi-sector configuration has been implemented using a plurality of 1FA 3-sector base station systems having the same FA.
FIG. 1 is a block diagram illustrating the case in which a 6-sector base station system is implemented using two conventional 1FA 3-sector base station systems.
As shown in FIG. 1, the 6-sector base station system includes a base station controller (BST) 10, a base station interconnection network (BIN) 20 for conducting a data interface with respect to the base station controller 10, and a base station control processor (BCP) 30 for controlling the overall operation of the base station controller 10. The system also includes a pair of 3-sector digital units, that is, a first 3-sector digital unit 40 and a second 3-sector digital unit 50, for processing and interfacing data between the base station controller 10 and each of sectors 81 to 86, a radio frequency unit (RU) 60 connected to both the first and second digital units 40 and 50 and adapted to process transmission and reception radio frequency signals, and a transmission medium (TM) 70 arranged between the radio frequency unit 60 and the sectors 81 to 86 and adapted to conduct a signal interconnection between the radio frequency unit 60 and the sectors 81 to 86.
The first digital unit 40 includes at least one shelf control and routing card assembly (SRCA) 41 for interfacing with the base station interconnection network 20, transmitting/receiving packet data of the digital unit 40, and collecting information about the status of the digital unit 40, at least one CDMA digital channel card assembly (CDCA) 42 for processing a CDMA digital signal, and at least one baseband and intermediate frequency conversion card assembly 43 (BICA) 43 arranged between the CDMA digital channel card assembly 42 and the RF unit 60 and adapted to conduct a digital interface with respect to the CDMA channel card assembly 42 while providing an interface with respect to the RF unit 60 at an intermediate frequency (IF) of 4.95 MHz.
Similarly to the first digital unit 40, the second digital unit 50 includes at least one shelf control and routing card assembly 51, at least one CDMA digital channel card assembly 52, and at least one baseband and IF conversion card assembly 53.
Each of the CDMA channel card assemblies 42 and 52 includes a controller and input/output (I/O) unit for conducting control and input/output operations, and 12 cell site modems (CSMs).
Practically, there are a plurality of shelf control and routing card assemblies 51 in one digital unit. Similarly, 10 CDMA channel card assemblies 52 are provided for one digital unit. There are also 3 baseband and IF conversion card assemblies 53 in one digital unit.
In the above mentioned conventional case, a multi-sector base station system, namely, a 6-sector base station system, is implemented by simply coupling together two conventional 3-sector base station systems having the same FA.
That is, the first digital unit 40 covers the sectors 81 to 83 whereas the second digital unit 40 covers the sectors 84 to 86. Accordingly, where a mobile station moves about within a zone covering the sectors 81 to 83 or within a zone covering the sectors 81 to 83, a softer handoff occurs. However, where a mobile station moves between the zone covering the sectors 81 to 83 and the zone covering the sectors 84 to 86, a soft handoff occurs. When a soft handoff occurs, it is possible to avoid a speech cut-off phenomenon because prior to a disconnection from the current base station, the mobile station is allocated with the channel element of a new base station so as to keep speech communications with the counterpart. In this case, however, a lengthened handoff processing time is taken. Furthermore, the efficiency of using channel resources is degraded because two channel elements are consumed.
Moreover, it is impossible to allocate idle channel elements, present in the first digital unit 40, for the sectors 84 to 86 covered by the second digital unit 50. Similarly, it is impossible to allocate idle channel elements, present in the second digital unit 50, for the sectors 81 to 83 covered by the first digital unit 40. For this reason, there is a problem in association with a flexible allocation of channel elements in the setting of a traffic channel.
As well known, conventional multi-sector base station systems have an advantage in that it is possible to implement a multi-sector base station system without any substantial hardware or software modification, in particular, in the case in which two 1FA 3-sector systems having the same FA are used.
However, there is a drawback in that increased costs are imposed for the construction of a desired system because a plurality of system units should be used. As mentioned above, there are also drawbacks of a lengthened handoff processing time and a degraded channel resource using efficiency because the soft handoff is made between 3-sector digital units. In addition, where a 6-sector BTS system is configured using two 3-sector 1FA digital unit shelves, there are drawbacks in that the area occupied by one base station device and the financial burden is increased because it is necessary to always use two 3-sector digital units for one base station device.
Therefore, an object of the invention is to solve the problems involved when a multi-sector (6-sector) base station system is configured using two conventional 1FA 3-sector base station systems having the above mentioned configuration, and to provide a switching device in a digital unit of a multi-sector base station, which switching device uses switching elements incorporated in the digital unit and adapted to switch 3 sectors into multiple sectors, thereby being capable of implementing a multi-sector base station system allowing a softer handoff among all sectors.
Another object of the invention is to provide a switching device in a digital unit of a multi-sector base station, which switching device uses switching elements adapted to support the switching of the next channel element stage in the digital unit included in the existing 3-sector base station to implement a base station system, thereby allowing all handoff between sectors to be conducted in a softer handoff fashion while allowing all channel elements to be set for all sectors covered by the base station upon setting a traffic channel, thereby achieving an efficient allocation of channel elements.
In accordance with the present invention, a 6-sector base station system is efficiently implemented in accordance with the setting of a timing capable of supporting an addition of switching elements, an operation of switching elements upon setting pilot, sync, and access channels, and an operation of switching elements upon a handoff and call setting.
In other words, there is a fundamental difficulty in implementing a multi-sector system capable of supporting a softer handoff between sectors in that channel elements are designed only to support a maximum of 3 sectors. In accordance with the present invention, switching elements are designed in order to allow data of a channel element, adapted to support only the existing xcex1, xcex2, xcex3-sectors, to be switched to further sectors while allowing data from multiple sectors more than 3 sectors to be switched to 3 sectors. By virtue of such a design, a multi-sector base station system is implemented.